[ADT] Make SmallSet::insert(const T &) return const_iterator

This patch makes `SmallSet::insert(const T &)` return
`std::pair<const_iterator, bool>` instead of
`std::pair<NoneType, bool>`. This will exactly match std::set's behavior
and make deduplicating items with SmallSet easier.

Reviewed By: dblaikie, lattner

Differential Revision: https://reviews.llvm.org/D131549